I want to push the component density limit. I'd like to place two 0402 components next to each other, with a toe to toe spacing (pad edge to pad edge) of 7 mils, which would provide 4mils solder mask webbing between the two pads. Any concern of solder bridging or insufficient solder? Is 4 mils enough SM webbing to isolate the two pads and solder paste?

7 mils seems very close to have a 4 mil mask sliver between the pads. The mask opening would only be 3 mils larger than the pad. There is a +/- .005" tolerance on solder mask registration. I would suggest using a solder mask swell that is 6 mils larger than the pad and spacing out the parts a little more. A solder mask sliver of 4-5 mils is ok.